1. Field of the Invention
The present invention relates to a paper feeding tray for a copying machine, printing machine or the like, and more particularly to an electric circuit thereof for generating GO and NO GO signals for the drawing of recording sheets from the tray.
2. Description of the Prior Art
A known elevator type paper feeding tray in a copying machine or printing machine or the like is constructed as shown in FIG. 1. Disposed within a tray frame 1 is a movable elevating plate 2 that ascends and descends in parallel with tray frame 1. Plate 2 descends to its lowermost position and stops there when a door (not shown) attached to tray frame 1 is opened as, for example, to enable restocking of plate 2 with sheets of transfer paper.
Located in the upper interior of tray frame 1 are a paper feeding roller 3 for feeding transfer sheets P, the shaft of which roller is disposed for vertical movability over a fixed range, a position detecting switch 4 for detecting the uppermost position of a pile or stack of transfer sheets P, and a photosensor 5 for sensing whether or not the tray contains transfer sheets P.
Switch 4 is a microswitch that performs position detection through contact between its actuator and the uppermost transfer sheet P.
Photosensor 5 is a detection element combinationally comprising a light emitting diode and a light receptive diode; the presence or absence of transfer paper on plate 2 is detected by reflection of light from the upper surface of transfer paper P. Thus, if there is no transfer paper P in position (in accordance with the operation of switch 4) and the light receptive diode detects light reflected from the surface of elevating plate 2, or if transfer paper P is fairly distant from photosensor 5, the resulting determination is that no transfer paper P is present in the tray.
Now, when the door is closed after transfer sheets or paper P has been placed on plate 2, the plate is elevated by a lifting mechanism (not shown) to its normal position in which the topmost sheet of transfer paper P comes into pressure contact with paper feeding roller 3--at which point elevation of plate 2 is discontinued.
When copying operations subsequently commence, the upper surface level of the stack of transfer paper P on plate 2 gradually descends as the transfer sheets are fed one after another by operation of paper feeding roller 3.
As the upper surface level of the transfer sheet stack drops below a prescribed level, microswitch 4 detects such lowered level (through a change in its operating state) and plate 2 is again elevated by the lifting mechanism until the upper surface of the transfer sheet stack has resumed its normal level. During this period, the feeding of transfer papers P continues by operation of paper feeding roller 3.
These operations are performed repeatedly. When the door is opened, however, paper feeding is halted and elevating plate 2 descends to and stops at the bottom of the tray.
The aforedescribed elevator type paper feeding tray is capable of being loaded with a large quantity of transfer paper--in some cases several thousands of sheets--and of continuous sheet feeding, making it a very convenient apparatus for use in a high speed copying machine.
However, in copying machines or the like having an elevator type paper feeding tray, the normally smooth paper feeding operations are disturbed when, after the door has been opened for one reason or another, further copying is attempted immediately following closing of the door. Put another way, paper feeding problems and smooth copying operations are likely to be interrupted when further copying is attempted before or within the period of time necessary to elevate the transfer sheet stack to its normal or prescribed level after the door is closed, or when copying is attempted despite the lack of transfer paper in the tray.